A so-called cyclone furnace is described in WO 03/002469. A mixture of a preheated combustion air and powdered coal (or other fuel) suspended in air, is fed into a combustion chamber. In the combustion chamber, the suspended particulate material/materials and air circulate, creating a cyclone circulation system (or approaching a creation of a cyclone circulation system). The suspended fuel is then combusted in the combustion chamber, in the presence of the suspended particulate mineral material. This is commonly referred to as a cyclone furnace. The suspension of coal in preheated air, and the particulate mineral material, is introduced through the top or close to the top of the combustion chamber.
This combustion should take place in the top of the combustion chamber, but if the air and fuel mixture is not correct, the combustion may take place in the exhaust pipe extending upward from the combustion chamber, which is undesirable as it causes the temperature to drop inside the combustion chamber and therefore reduces the efficiency of the mineral melting process. In order to control this process it is found that an accurate feeding of the fuel, such as powdered coal, is very important.
To ensure a constant feed of powdered coal, this feeding must be gravimetrically constantly dosed so that the right mixture of combustion fuel (such as powdered coal) and oxygen is controlled constantly.
The fuel in the form of powdered coal is accommodated in a one or more storage container, from which batch filling of one or more dosing containers takes place. A continuous material feed then is carried out, from at least one dosing container, for further processing of the material. As described in U.S. Pat. No. 5,670,751, the dosing container is supported on weighing cells. However, depending on the amount of material which is present in the storage container, the pressure differences in the system, or other factors, that can affect the weighing cells, the measurement of the weighing cells may be influenced and thereby become inaccurate. In order to compensate for this, a flexible tube compensator is provided on the feeding pipe between the storage container and the dosing container and another compensator is provided on the discharge line from the dosing container. Besides these measures, an electronic metering control is provided in the system disclosed in U.S. Pat. No. 5,670,751. A similar solution is known from EP 0 123 777. Although such correction means can reduce the problem of inaccurate weighing measurements, it has in some applications of gravimetric bulk material feeding, such as the feeding of powdered coal to a cyclone furnace, proven insufficient, in particular during the refilling of the dosing container(s), as both the storage container and the dosing container have some external displacements as the weight of the contents change.